A pipe coater experienced cracking issues in the top layer of dual layer fusion bonded epoxy systems (FBE) during large scale bending tests at low temperature. White spots were observed on most damaged top coats. The customers asked for technical support of MSC to discover the root cause of the cracking.
The composition of the powders was analysed using X-ray fluorescence (XRF) and electron probe microscopy (EPMA). Their curing behaviour was determined by means of modulated differential scanning calorimetry (MDSC). From a coated sample, cross sections were taken after different bending and reheat conditions for analysis by optical and electron microscopy.
Analysis showed little influence from powder composotion or curing behaviour on sensitivity to cracking. However, the reheating speed after bending seems a highly significant factor in the appearance of cracking.
White spots show to be the preferred crack initiation location. Analysis of the composition of white spots indicates cross contamination during coating application.
Therefore the customer was advised to select a better adhering fusion bonded epoxy (FBE) and to optimise cleaning efficiency. Since, the pipe coater no longer experiences cracking issues.
EPMA image showing area around white spots
Morphology at crack
Cross section showing crack in top layer